Iconic colorimetric test device with reduced susceptibility to false positive and false negative readings

Abstract

Laminated test devices in which positive results are detected by the appearance of a symbol on the surface of the device due to a color change in an indicator in the device are improved by placing the indicator lamina on one side of a light-transmissive substrate sheet and a pigment lamina on the other side, the indicator lamina forming the icon and the pigment lamina forming the background, the pigment being the color of the indicator prior to the color change. This arrangement reduces false positive readings by obscuring the outlines of the symbol prior to the color change. False negative readings are reduced by further changes in the arrangement of the laminae.

1. A test device for analyzing a liquid sample for the presence of a selected analyte, said test device comprising: a light-transmitting, liquid-impermeable substrate sheet coated on one side with an analyte indicator lamina and on an opposing side with a pigment lamina, said analyte indicator lamina reactive with said analyte in a reaction producing a color change in said analyte indicator lamina, and said pigment lamina being of the same composition as that of said indicator lamina, so that said pigment lamina is of a color visually indistinguishable from that of said analyte indicator lamina when said analyte indicator lamina is in unreacted condition, said pigment lamina fully covering a selected test region on said substrate, and
 means for restricting said color change in said analyte indicator lamina to a restricted area within, and smaller than, said test region, thereby leaving adjacent areas within said test region unchanged. 
 
     
     
       2. The test device of  claim 1  wherein said analyte indicator lamina fully covers said selected test region, and said means for restricting said color change is a channeling lamina overlying said analyte indicator lamina, said channeling lamina having an opening therein to expose said restricted area of said analyte indicator lamina. 
     
     
       3. The test device of  claim 2  wherein said channeling lamina is transparent. 
     
     
       4. The test device of  claim 1  wherein said means for restricting said color change is the application of said analyte indicator lamina only to said restricted area, leaving said adjacent areas free of said analyte indicator lamina. 
     
     
       5. The test device of  claim 1  wherein said substrate sheet is transparent. 
     
     
       6. The test device of  claim 1  wherein said restricted area is in the shape of a plus sign, and said adjacent areas surround said plus sign contacting all boundaries thereof. 
     
     
       7. The test device of  claim 1  further comprising a control indicator lamina that is reactive with said sample in a reaction producing a color change in said control indicator lamina regardless of whether said analyte is present in said sample, and means for restricting said color change in said control indicator lamina to a restricted area separated from said restricted area of said color change in said analyte indicator lamina. 
     
     
       8. The test device of  claim 1  wherein the side of said pigment lamina opposite said substrate sheet is coated with an opaque white lamina. 
     
     
       9. The test device of  claim 1  wherein said analyte indicator lamina is a hydrophilic fluid-permeable polymer impregnated with a compound containing an ionizable phenol group and a negatively charged group, said polymer containing quaternary ammonium groups in sufficient quantity to immobilize said compound against diffusion when wetted with an aqueous liquid sample and to cause said reaction to occur at a pH equal to or greater than a transition point within the range of 4.6 to 4.8. 
     
     
       10. The test device of  claim 9  wherein said compound is a member selected from the group consisting of bromophenol blue, bromochlorophenol blue, bromocresol green, bromocresol purple, bromothymol blue, brilliant yellow, and nitrazine yellow, and said quaternary ammonium groups are tri-(C 1 -C 4  alkyl)ammonium groups. 
     
     
       11. The test device of  claim 9  wherein said compound is nitrazine yellow and said quaternary ammonium groups are trimethylammonium groups. 
     
     
       12. The test device of  claim 1  wherein said analyte indicator lamina is a matrix of material permeable to gas but impermeable to aqueous liquids, impregnated with an compound that undergoes a color change upon contact with amines, and said test device further comprises a lamina of solid alkali. 
     
     
       13. The test device of  claim 12  wherein said compound is a member selected from the group consisting of bromocresol green, bromophenol blue, bromocresol purple, bromochlorophenol blue, and nitrazine yellow. 
     
     
       14. A test device for analyzing a liquid sample for the presence of a selected analyte, said test device comprising: a light-transmitting, liquid-impermeable substrate sheet coated on one side with an analyte indicator lamina and on an opposing side with a pigment lamina, said analyte indicator lamina reactive with said analyte in a reaction producing a color change in said analyte indicator lamina, and said pigment lamina being of the same composition as that of said indicator lamina, so that said pigment lamina is of a color visually indistinguishable from that of said analyte indicator lamina when said analyte indicator lamina is in unreacted condition, said pigment lamina fully covering a selected test region on said substrate, and means for restricting said color change in said analyte indicator lamina to a restricted area within, and smaller than, said test region, thereby leaving adjacent areas within said test region unchanged;
 and further comprising a control indicator lamina that is reactive with said sample in a reaction producing a color change in said control indicator lamina regardless of whether said analyte is present in said sample. 
 
     
     
       15. The test device of  claim 14  comprising a means for restricting the control indicator color change in said control indicator lamina to a restricted area separated from said restricted area of said color change in said analyte indicator lamina. 
     
     
       16. The test device of  claim 15  wherein said analyte indicator lamina fully covers said selected test region, and said means for restricting said color change in said analyte indicator lamina is a channeling lamina overlying said analyte indicator lamina, said channeling lamina having an opening therein to expose said restricted area of said analyte indicator lamina. 
     
     
       17. The test device of  claim 16  wherein said channeling lamina is transparent.